1. Field of the Invention
The invention relates to a protection means for objects included in an electrical power supply network.
2. The Prior Art
In Nimmersjo application Ser. No. 493,915, filed Aug. 1, 1974, which issued as U.S. Pat No. 3,956,671 FIGS. 2 and 3 show a proposal for level and sequence detection in a directional wave detector. This solution can at least theoretically provide an unselective tripping in certain cases. If we assume that a power line connects three consecutively positioned stations A, B and C, there will be two line paths A-B and B-C. Each station has two protection means, each one sensing a fault which occurs but giving a tripping signal to its breaker only if the fault is detected as lying ahead. The station B, lying between A and C, has a protection means B1 which supervises the distance B-A and a protection means B2 which supervises the distance BC. A fault at a point a on the line between A and B will therefore be detected by the protection means B1 as a fault lying ahead, whereas the protection means B2 detects the same fault as a fault lying behind. In station C, the protection means C1, which supervises the distance C-B, will detect the fault as a fault lying ahead, since it lies outside the range of protection of the protection means but in its direction of supervision. The protection means in station B are furthermore arranged, in the event of a fault lying behind, to emit a blocking signal to the opposite protection devices A2 and C1, respectively, so that these do not emit a tripping signal to the breakers. In the case mentioned above, the protection means B2 is thus supposed to emit a blocking signal to the protection means C1 so that this latter protection means does not emit a signal for disconnection of the line C-B. This is explained in the last paragraph of page 4 of the patent.
The object of the level detector NB in FIG. 2 in the patent is to block the blocking signals which are below a certain level. Because of reflection in station C of the voltage wave which emanates from the source a of the fault, it is theoretically possible that such a great change in voltage may appear in station C that the level of the output signal from the wave detector C1 in C, which is in the tripping direction, will be greater than the blocking level of the wave detector B2 in B. It is therefore theoretically possible that the level for tripping can be exceeded in C without the level for blocking being reached in B, which means that the line B-C will be incorrectly disconnected upon a fault in the line A-B.